Description of the Prior Art
The conventional optical detector is a two terminal device such as a PIN diode that is not electrically controllable. In an article entitled "Optical Response of the GaAs MESFET at Microwave Frequencies and Applications", by the inventors of this application along with A. Madjar and Thomas Higgins, which was published in the June 1991 issue of the Journal of IEEE, it is suggested that the optical sensitivity of a MESFET detector can be improved by separating the source and drain so as to permit more light to enter the device, and in an article entitled "Optically Controlled GaAs MMIC Switch Using a MESFET as an Optical Detector", by A. Paolella, A. Madjar, P. R. Herczfeld and D. Sturzebecher, in the June 1990 issue of the Journal of the IEEE it suggests that a density of 10.sup.17 carriers/cm.sup.3 be used.
As explained in the first article, a light detector device is comprised of a channel parallel to a surface thereof that is doped, usually with N type carriers, and a depletion layer or barrier between the channel and a substrate. Spaced source and drain electrodes are mounted in electrical contact with the channel, and a gate electrode is mounted between them. An increase in light energy impinging on the channel in areas between the electrodes causes the channel to become deeper so as to increase its cross section and reduce its resistance, thereby increasing the current flowing between the source and drain electrodes for any given bias voltage between the gate and source electrodes. Thus, variation in the light energy causes a variation in the current produced by the FET.
The concentration of doping in FET amplifiers is not greater than 10.sup.16 carriers/cm.sup.3. This again can be controlled by the gate voltage. Although greater concentrations would increase the gain of an amplifier, its output impedance would eventually become so low as to decrease the voltage swing. Furthermore, too high a current density would result from the fact that the depth of the channel decreases with the increase in concentration, and thus could lead to destruction of the amplifier.
U.S. Pat. No. 4,859,965, issued on Aug. 27, 1989, shows a MESFET amplifier in which the gain is changed by varying the illumination light energy on the device.